Electron Spin Control of Nanoparticles Could Advance Sensor Technology

Photonics SpectraOct 2016
A technique used to detect and control the electron spin resonance (ESR) of nanodiamonds in a vacuum chamber may lead to the development of novel sensors for detecting, measuring and monitoring gases. It may also provide a future template for the testing of quantum physics at the macroscopic level using nanoparticles.

Researchers at Purdue University demonstrated the electron spin control and direct temperature measurement of nitrogen vacancy centers (NVCs) in nanodiamonds, which they optically levitated in a low vacuum using a 1,550-nm laser.

A schematic of an optical tweezer used in a vacuum chamber by Purdue University researchers, who controlled the "electron spin" of a levitated nanodiamond. Courtesy of Purdue University/Tongcang Li.
One type of laser was used to trap and levitate the nanoparticles in a vacuum chamber, and another was used to monitor the electron spin. A millimeter-scale antenna delivered microwaves to control and flip the electron spin, and a spectrometer was used to detect changes in spin. A vacuum was used to reduce interference from air molecules.

Levitating the nanodiamonds in a vacuum enabled precise control and rigorous measurement of the floating particles. The nanodiamonds, which were approximately 100 nanometers in diameter, contained NVCs (atomic-scale defects formed in the diamond lattice by substituting a nitrogen atom for a carbon atom and creating a neighboring void in the crystal lattice). The researchers exploited this feature to control the electron spin.

They studied the effect of the spin under different conditions and found that the ESR contrast of an optically levitated nanodiamond was enhanced in a vacuum environment.

The researchers attributed the enhanced ESR to a reduction in low-quality negatively charged NVCs near the surface, due to the reduction of oxygen surface termination and a moderate increase in the temperature that quenches low-quality surface NVCs without significantly affecting high-quality NVCs at the the nanodiamond’s center.

The research team also observed that oxygen and helium gases had different effects on the photoluminescence and ESR contrast of nanodiamond NVCs. These effects were found to be reversible, indicating that nanodiamond NVCs could potentially be used for a sensor to detect and measure gases. Nanodiamond-based sensors represent a potential improvement over conventional sensors.

A nanodiamond levitated in a vacuum chamber developed by Purdue University researchers, who controlled its "electron spin." Courtesy of Purdue University/Thai Hoang.
"We've shown how to continuously flip the electron spin in a nanodiamond levitated in a vacuum and in the presence of different gases," said professor Tongcang Li. "While more detailed studies are required to fully understand this phenomenon, our observation suggests a potential application for oxygen gas sensing."

The levitating nanodiamonds may also be used in quantum information processing, in experimental techniques to probe fundamental physics in quantum mechanics, and in the measurement of magnetic and gravitational fields.

The results of the experiment pave the way toward a levitated spin–optomechanical system for studying macroscopic quantum mechanics.

The use of atoms, molecules and molecular-scale structures to enhance existing technology and develop new materials and devices. The goal of this technology is to manipulate atomic and molecular particles to create devices that are thousands of times smaller and faster than those of the current microtechnologies.

A small object that behaves as a whole unit or entity in terms of it's transport and it's properties, as opposed to an individual molecule which on it's own is not considered a nanoparticle.. Nanoparticles range between 100 and 2500 nanometers in diameter.